Catalytic Performance Of LDHs Supported Gold Catalysts

Gold catalysts have been widely studied in the recent 30 years due to their excellent catalytic activity at low temperature. The catalytic activity of gold catalysts supported on metal oxides and mesoporous molecular sieves can easily reach 100% for CO oxidation at low temperature. However, the low stability of gold catalyst becomes a bottleneck in its commercial application. Therefore, preparation of gold catalysts combined with good stability and high catalytic activity and carriers became a hot issue.In this work, the gold catalyst was prepared through the gold colloids immobilization method and ion-exchange of liquid phase assembly method, in which the synthesized layered double hydroxides (LDHs) was used as the carrier. The effect of pH on the adsorption of gold colloids, the dosage and type of the reducing agent, the loading of gold, the crystallization time of LDHs, LDHs with different compositions on their activity and stability were studied systematically. The catalyst samples were characterized by XRD, TEM, and XPS et al. The activity and stability of the catalysts were tasted by CO oxidation reaction. The results showed that:(1) Nano-gold catalysts with high activity was prepared by adsorption of the gold sols with a loading ratel-2wt% to the LDHs carriers through electrostatic attraction, which is simple, effective and repeatable; and the CO conversion rate could reach 100% by using the catalysts.(2) When the loading rate of gold catalyst was at the theoretical loading rate(1?2wt%), a weakly acidic to neutral pH environment was beneficial to the loading of the gold sols on the LDHs carrier, at which the prepared gold catalyst contain gold approaching to the theoretical loading rate.(3) By setting the theoretical loading rate at lwt%, it was found that the conversion of CO on 1% Au/MgAl-LDHs?1% Au/ZnAl-LDHs and 1% Au/MgFe-LDHs gold catalysts achieved 100% at room temperature, and the size of the gold particles on the surfaces of the three samples were 1-5nm.(4) Due to the difference on the stability of the three samples supported on LDHs the above mention, the conversion of CO decreased to 50% after continuous catalytic reaction for 266h,195h and141h. One of the main reasons for the catalyst deactivation was that the crystal structure of LDHs was unstable. Gold particles were covered by the collapse of the carriers, which decreased the content of gold presented on the surface of the catalyst. In addition, the interaction between LDHs and nano-gold was impaired so that the gold particles transferred, reunited and grew.(5) Gold catalyst was prepared by ion-exchange of liquid phase assembly method by using gold chloride acid as the precursor. The catalytic activity was improved by improved by increasing the amount of reducing agent NaBH4; and the more activated catalyst was synthesized by using THPC as the reducing agent.(6) The nano gold particle were intercalated and assembled in ZnAl-NO3-LDHs and MgAl-NO3-LDHs, but the two catalysts kept different activities, the latter was more active.